A cyst is a closed sac, having a distinct membrane and division compared to the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (not unlike the manner in which water molecules group together, forming a bubble); however, the distinguishing aspect of a cyst is that the cells forming the "shell" of such a sac are distinctly abnormal (in both appearance and behaviour) when compared to all surrounding cells for that given location. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst may sometimes resolve on its own. When a cyst fails to resolve it may need to be removed by surgery but this will depend on what type of cyst it is and where in the body it has formed.

This is just one example of how the Greek root cyst-, which simply means a fluid-filled sac, is also found in medical terms that relate to the urinary bladder and the gallbladder but that have nothing to do with cysts.